For me, the mobile theme would still load on iPad when using "iPhone|iPod|Mobile" as the expression.
In the end I created two exceptions for each section as they work like an if / elseif statement.
First match was to match the devices I wanted the desktop theme to load for e.g. iPad|Tablet to load "default". The second match was for the other devices e.g. "iPhone|Mobile" to load the mobile theme.